1. Field of the Invention
This invention relates to printed wiring boards. More particularly it relates to printed wiring boards designed for wave soldering operations. Printed wiring boards are also known as printed circuit boards.
2. The Problem Solved by the Invention
Printed circuit boards (PCB's) are most laminates of glass-filled epoxy insulating layers and layers of etched copper conductors. The simplest interconnection structure consists of a single layer of insulation made of a glass-filled epoxy and a single layer of copper conductors. More complex structures consist of numerous layers of conductors and insulators which are divided into power distribution layers, signal layers, and ground plane layers. As used in this disclosure, the terms printed circuit board and printed wiring board (PWB) are synonymous.
In integrated structures such as those associated with semiconductor circuits, the first level of packaging consists of a number of switching circuits fabricated on a single part called a chip. The chip is mounted in a structure called a module, which provides environmental protection for the chip and permits electrical interconnection to be made to the next level of package. Modules are normally provided in standard configurations to allow for the design of general-purpose manufacturing equipment and higher-level packages. A particularly common standard configuration is the dual in-line pin ("dip" or "d-i-p") package. Such a module is substantially rectangular as viewed from above and has two rows of electrical leads projecting from opposite edges of the module. The lead spacing is typically 0.100" on center. Most commonly the leads will have a 90.degree. bend in them such that the leads project downward from the plane of the chip.
Circuit modules are most often mounted to PCB's by inserting the component leads into holes in the circuit board which extend from one side of the PCB (the "component side") to the other side of the board ("solder side" or "conductor side"). Surrounding each hole on the solder side of the circuit board is a solder pad. The leads are mechanically and electrically connected to the circuit board by soldering each lead projecting through its designated hole in the circuit board to the solder pad which surrounds the hole. Such soldering may be accomplished either manually or by machine soldering processes. Probably the most common type of machine soldering process is wave soldering.
FIG. 1 depicts a typical wave soldering process for printed circuit boards. Molten solder is pumped up and over support plates to form a wave. A printed circuit board carried on a conveyor is passed over the solder wave at an angle such that the solder side (conductor side) of the board contacts the leading edge of the solder wave. Component leads are soldered to the solder pads on the wiring board and plated through-holes fill with solder by capillary action.
If two clean metal surfaces are held together and dipped into molten solder, the solder will wet the metal and climb up to fill the gaps between the adjacent surfaces. This phenomenon is the result of capillary action. Wave soldering processes rely on this phenomenon to fill plated through-holes and produce a fillet of solder both on the upper surface (component side) of the circuit board and surrounding the portion of the lead which projects through the plated through-hole to the solder side of the circuit board. This results in a greater area of electrical contact between the component leads and the conductors of the printed wiring board thereby decreasing the electrical resistance and increasing the mechanical integrity of such connections.
This same phenomenon, however, can produce unwanted solder bridging between closely spaced component leads. The trend towards higher density circuit boards (i.e., the closer spacing of components on printed wiring boards) has made this a matter of particular concern to electronic manufacturers. Generally, the problem is non-existent or minimal for components having a lead spacing of 0.100" or more. However, when lead spacing is reduced below about 0.070", the problem becomes noticeable. At a lead spacing of 0.050", the problem is significant.
Many circuit board components, particularly connectors for ribbon cable and the like, now have leads with 0.050-inch spacing. More and more components with such closely spaced leads are likely to be used in the future as the trend to higher and higher circuit board density continues.
The problem is illustrated in FIGS. 2 and 3. An electronic or electrical component (such as a ribbon cable terminal) is mounted on a printed wiring board with its leads projecting through the board. Each lead hole in the board is surrounded by a solder pad on the solder side of the circuit board. The circuit board is then passed through a wave soldering machine as illustrated in FIG. 1. The direction of the board's passage through the solder wave is indicated by the arrows in FIGS. 2 and 3.
As the board passes through the solder wave a solder fillet forms around each lead connecting the lead to the surrounding solder pad. For closely spaced leads, a bridge of molten solder also forms between adjacent leads. It is believed that this solder bridge wicks from one pair of adjacent leads to the next (downstream) pair as successive leads pass through the solder wave. Thus, this solder bridge is a transient phenomenon for most of the leads. However, when the last (downstream) lead of the component passes through the solder wave, there is no adjacent downstream lead to wick the solder bridge further downstream. It therefore remains established between the last two leads in the array as shown in FIG. 3 and solidifies as the board exits the solder wave.
In the past it has been necessary to manually remove such solder bridges by reheating the connections and removing the excess solder with a vacuum device such as a solder sucker or with a copper braid or the like (which wicks molten solder). This is a time-consuming process and it has the further disadvantage of weakening the solder connections to the printed circuit board since each remelting of a soldered joint degrades such a connection.
In the alternative, solder bridging may be mechanically removed by scraping, cutting, or the like. Again, this is a time-consuming process which has a significant probability of damaging the electrical and/or mechanical integrity of the connection.
The present invention solves this problem.